March 5, 2018

Evaluating Incubation Substrate

Many of us probably learned back in elementary school about the importance of gravel to salmon and trout for building redds (nests) and incubating eggs. Such benthic substrate – the material at the bottom of lakes or rivers on which organisms live and grow – is crucial to many freshwater fish species around the world for the incubation and rearing of eggs and larvae. While it’s true that it might seem like simply a way to keep eggs from drifting down the river, and keeping alevin (newly hatched fish) away from predators, that’s only part of the story.

The gravel, or rearing substrate, also provides developing alevin with an optimal energy-saving environment. Until it can feed for itself, an alevin’s yolk sac is its entire food supply. The yolk provides all its energy for growth, development, and metabolism. The amount of yolk is finite: the more energy that goes to one process, the less that is available for others. Because alevin move less in substrates, more yolk is available for growth.

Rearing substrate also plays an important role in the survival and optimal growth of larval white sturgeon. Sturgeon larvae have a large yolk sac, and are poor swimmers. When nestled amongst substrate, like salmon and trout, they swim less and are able to put more of their energy into growth. In the wild, substrate would also likely give these newly hatched fish a safe place to hide, with the added bonus of lots of invertebrates close-by for food.

While the benefits of using substrate to rear newly hatched trout and sturgeon are clear, its use is still fairly uncommon in most hatcheries. The Society was among the first to use substrate with white sturgeon, outside of research, in our full hatchery production – significantly increasing the survival rates of fish that we culture for conservation.

Sturgeon larvae without substrate and with substrate

Various forms of rearing substrates have been used, somewhat infrequently, in salmon and trout hatcheries since the mid-1970s. Research showed that substrate incubation produced larger fry at first feeding, and reduced mortality. Substrates used in hatcheries have varied from do-it-yourself materials (like rolled-up Vexar plastic-mesh netting and filter media) to commercially produced products.

This past spring, we began an evaluation of a substrate at the Fraser Valley Trout Hatchery. This evaluation was to quantify any benefits of using substrate, and to note any operational and logistical constraints. Essentially, how does this substrate perform in our facilities, in our tanks, and with our fish? If it provides a benefit, how much better is it than conventional rearing?

The results showed that rainbow trout reared in substrate for only 20 days had significantly improved rates of growth that persisted right until the end of the evaluation. We found that substrate-reared fish reached a weight of one gram 14 days before fish reared under standard conditions. The reasons for this are varied, but centre around the concept of providing the best conditions early in life for optimal growth and health. Many are familiar with the notion that providing children with a high-quality, nurturing environment can lead to a head start in life; this appears to be similar with fish. Recent studies have also shown that steelhead reared in substrate had greater brain size and development than those reared without substrate. 

We’re excited about these results, and are working to better understand where and when substrate rearing would be beneficial in our hatchery programs. In addition, we are trialling the use of substrate in existing incubators as well as in dedicated substrate incubators.

Author: Marcus Boucher; Freshwater Fisheries Society of BC
Photo Credit: Marcus Boucher; Freshwater Fisheries Society of BC